Especially for current measurement, it is normal to use a shunt, which is essentially in the form of a defined resistance. It is normal to indicate the voltage drop across this resistance as the current value. A shunt such as this is frequently at a potential higher than ground potential.
Current measurement on a shunt which is at a potential is known. The LM 3822 module from the National Semiconductor Company produces a PWM signal after analog/digital conversion, for current measurement (see Internet www.Power.National.com).
When a shunt such as this is at high potential, DC isolation is generally specified for further processing of the measurement signals. Optocouplers, for example, are used for this purpose. Since the voltage drop is generally amplified by operational amplifiers or the like, an auxiliary power supply is also necessary, at least for the amplifier. Thus, in practice, it is necessary to accept significant complexity for the auxiliary power supply on the one hand and for the voltage decoupling on the other hand, for this purpose.
A linear current measurement circuit is known from DE 199 10 755 A1, especially for a motor circuit in which an input circuit which is at high voltage and has an amplifier and an A/D converter supplies a digital signal with a sequence of current pulses, which are converted back to an analog signal at ground potential.
Furthermore, FR 2 541 777 A discloses an apparatus for signal transmission by way of transformers between a primary section in the low-voltage range and a secondary section in the high-voltage range. A power supply is provided in the primary section, which transmits the energy which is required for operation of the secondary section by way of a transformer, in the form of power pulses from the primary section to the secondary section. In this case, the information is transmitted by the duty ratio of the periodic power pulse pauses.